Based on our previous data and current results, we investigated whether Notch3 and PTEN inhibited the ability of breast cancer cells to proliferate and found that activated Notch3 induced PTEN in vitro, and high Notch3 and PTEN expression levels were associated with a weaker ability to proliferate, suggesting that these pathways, at least in part, may be involved in the inhibition of tumorigenesis. The gene discussed is NOTCH3; the disease is breast cancer.